The invention relates to a process for the production of bristle articles, comprising a bristle carrier and thermoplastic material bristles anchored therein, in that holes for receiving the bristles are formed on the bristle carrier, the fixing-side ends of the bristles are melted, accompanied by a shortening of their length and the formation of a thickened portion and are inserted with the thickened portion first into the holes on the bristle carrier.
Following the discovery of thermoplastics and the use thereof in bristle articles, numerous attempts have been made to utilize the thermoplastic characteristics of such materials for fixing the bristles to the bristle carrier. In particular, the fixing-side end of the bristle bundles has been melted, so that a thickened portion is formed and is then used for fixing in the bristle carrier. Thus, for example, it is known from U.S. Pat. No. 2,664,316 to produce a paint brush in that the bristle bundle is inserted end is melted from the back. Subsequently the melted material of the bristles is displaced from the back into the undercut. Following cooling the bristle bundle is firmly anchored in the carrier.
In another known process described in, for example, U.S. Pat. No. 3,604,043, the fixing-side end of the bristle bundle is cylindrically shaped during the melting process, its external diameter corresponding to that of the holes in the bristle carrier. The holes are widened in the vicinity of the bottom to form undercuts. The still plastic end of the bristle bundle is inserted under pressure into the hole, so that the plastic mass can flow into the undercut.
A similar process proposed in DE-A-34 00 510 and U.S. Pat. No. 4,609,223, is based on a bristle carrier with bead-like projections on the edges of the holes, in which initially the bead-like projections projecting over the fixing side of the bristle carrier are smooth and then shaped inwardly into the holes, so that undercuts are formed. The still plastic, fixing side ends of the bristle bundle are then introduced under pressure into said holes, so that the plastic material fills the hole.
The butt welding of bristle bundles onto plastic bristle carriers is also known from EP-A-O 149 996 and corresponding U.S. Pat. No. 4,609,228, as is the injection molding of the fixing side thickened portions of bristle bundles in the plastic material of the bristle carrier. EP-A-O 142 885 and corresponding U.S. Pat. No. 4,635,313 and DE-C-35 11 528 and corresponding U.S. Pat. No. 4,892,698 also describe such injection molding processes. Finally, another known process in, for example, DE-A-O 34 03 341 and corresponding U.S. Pat. No. 4,637,660 makes use of the special characteristics of plastics, namely the molecular reorientation occurring on heating, in that the thickened portion is pressed onto the fixing side ends of the bristle in a planar bristle carrier, whose surface is melted. The bristle carrier melt flows together behind the thickened portion, because as a result of molecular reorientation the plastic attempts to reassume its original shape. The latter process has in particular made it possible for the first time to obtain a joint similar to the produced by welding, in which the bristle carrier and bristles can be made from different plastics.
It is finally known from DE-A-36 37 750 only to melt the holes in the bristle carrier and in particular the bottom thereof and to introduce the free bristle ends into the fluid melt, so that it rises between the bristles and between the latter and the wall of the hole. On cooling the bristles are anchored by the penetrated bristle carrier material.
All the aforementioned processes suffer from the common disadvantage that, as a result of the processing of the bristles and/or the bristle carrier close to or above the flow or melting point or temperature or, in the case of partly crystalline thermoplastics, the crystalline melting temperature or point, cooling takes a considerable time until an extraction-resistant joint is obtained, which leads to long machine cycle times.
The aim underlying the present invention resides in providing a process, in which it is possible in a minimum time to join bristles made from a thermoplastic material to random bristle carriers.
According to the invention the holes in the bristle carrier are constructed with an at least zonally smaller free cross-section than the cross-section of the thickened portions at the bristle ends and that the thickened portions and/or the wall of the holes are heated to a joining temperature between the transition temperature and the flow temperature in the case of amorphous thermoplastic and between the latter and the crystallite melting temperature in the case of partly crystalline thermoplastics or are kept at said joining temperature in which the thermoplastics are in the entropy elastic state and in this state the thickened portions are inserted by thermoforming into the holes.
Contrary to the prior art, the invention uses for the joining process the so-called entropy elasticity (thermoelasticity) of all thermoplastic materials above the temperature (transition temperature), at which the molecular orientation freezes, but below the temperature (flow temperature in the case of amorphous or crystallite melting temperature in the case of partly crystalline thermoplastics), in which the molecular union has dissolved to such an extent that the thermoplastic starts to flow and cannot molecularly re-form, instead of the plastic behavior of the plastic material above the crystallite melting or flow temperature. As a result of its molecular reorientation, on cooling the thermoplastic attempts to reassume the original shape. The invention makes use of the shape change which occurs for joining the bristles and the bristle carrier. The following procedure is adopted.
Bristles in monofilament or multifilament form are initially melted at their fixing side ends. In known manner, the originally stretched molecular structure passes into a convoluted molecular structure which, accompanied by the shortening of the bristle length, leads to a thickening at the fixing side end. This structure is maintained on cooling, the transition temperature (also called "softening temperature" on heating) being above ambient temperature in the case of amorphous thermoplastics and below it with partly crystalline thermoplastics. If the bristles are introduced by the thickened portion at an elevated temperature, which is above the transition temperature and preferably just below the flow temperature (in the case of amorphous thermoplastics) or in the vicinity of the crystalline melting temperature (in the case of partly crystalline thermoplastics) are inserted in the holes on the bristle carrier under pressure or tension, then on traversing the cross-sectional reduction of the hole and accompanied by the building up of internal stresses, the thickened portion undergoes a shape change as a result of the thermoelasticity of the plastic. On cooling, the plastic attempts to reassume the original shape of the thickened portion (relaxation)in the vicinity thereof, accompanied by the reduction of the internal stresses, but is prevented from doing so by the cross-sectional reduction. As a result the internal stresses are at least partly retained and are used as clamping forces for the bristles in the bristle carrier hole. This effect occurs independently of the bristle carrier material, so that random materials can be used for this purpose. If the bristle carrier is made from a thermoplastic, then only the wall of the hole need be heated to the elevated temperature. In this case the relaxation of the hole wall ensures the fixing of the bristles or the thickened portion thereof. In the case of this pair of materials both can be raised to the joining temperature, i.e. both the thickened portion at the bristle ends and the hole wall. A thermoforming process is involved in all cases.
This leads to a firm seating of the bristles in the bristle carrier. The major advantage of the inventive process is that the melting of the bristle ends for forming the thickened portion can take place in a process stage separate from the actual joining process and the actual joining or connection can be performed at a lower temperature than the flow temperature, so that for the final cooling to an adequately low handling temperature, it is necessary to overcome a much smaller temperature difference and there is no need for the otherwise necessary removal of the melting heat, which leads to a significant reduction in the machine cycle time for the actual joining process.
The decisive temperatures for certain thermoplastics are given below:
______________________________________ Partly Crystalline Thermoplastics Softening or (glass) Crystallite melting transition temperature temperature ET KT ______________________________________ PE -125.degree. C. +135.degree. C. PP -20.degree. C. +165.degree. C.
6,6 +57.degree. C. +265.degree. C. ______________________________________ Amorphous Thermoplastics Softening or (glass) Flow or melting transition temperature temperature ET FT ______________________________________ PVC +80.degree. C. +170.degree. C. PS +90.degree. C. +180.degree. C. ______________________________________
The joining temperature of bristles and bristle carrier in the case of a thermoplastic/thermoplastic material pairing of said elements will preferably in the range of the lower KT or FT of the two joining partners and will mainly utilize the shape change thereof. Thus, e.g. in the case of bristles made from PA (polyamide) with a very high crystallite melting point of 265.degree. C. and a bristle carrier of PP (polypropylene) and with a joining temperature close to its crystallite melting point of 165.degree. C. an excellent anchoring of the bristles is obtained. However, if the bristle carrier is made from a rigid material having no thermoplastic behavior in the temperature range under discussion here, then the joining temperature must be fixed in accordance with the participating thermoplastic of the bristles, i.e. about 265.degree. C. in the case of PA. In general, it can also be stated that the maximum shape changes are obtained with partly crystalline thermoplastics close to the KT, whereas with amorphous thermoplastics this is in the middle range between ET and FT.
According to an embodiment of the invention the thermoplastic used for the bristles has a greater deformation resistance at the joining temperature than the thermoplastic used for the bristle carrier. Thus, as in the aforementioned material example, in this case substantially only the shape change and relaxation of the bristle carrier material is used.
Instead of this, according to another embodiment, a thermoplastic can be used for the bristle carrier which has a higher deformation resistance at the joining temperature than the thermoplastic used for the bristles. In this case the shape change is essentially limited to the thickened portion at the fixing side ends of the bristles.
It has been found that in the case of the process according to the invention, it is also possible to use bristle carriers from completely different materials, e.g. wood or thermosetting plastics. A completely satisfactory anchoring of the bristles still occurs and a self-closure is always obtained. With a rigid bristle carrier, e.g. of wood or thermosetting plastic, surface roughness into which the thermoplastic material of the bristles penetrates by relaxation is sufficient as the cross-sectional reduction.
It has also proved advantageous if the joining process is performed at such a high speed, that the joining time is below the relaxation time of the participating thermoplastic or thermoplastics. The invention makes use of the fact that the deformation resistance also increases with increasing deformation speed, i.e. the joining partner appears to be harder than it would be as a result of its temperature. This e.g. makes it possible to insert the thickened portion on the bristle ends in the bristle carrier a relatively short time after melting.
According to another preferred embodiment of the invention, the holes on the bristle carrier can be given a larger free cross-section in the vicinity of the edge of the hole than the thickened portion on the bristle ends and following this can have a cross-section decreasing to the narrowest cross-section.
As a result of the larger free cross-section of the holes in the vicinity of the edge thereof and the constant reduction to the narrowest cross-section, a type of insertion bevel is formed, which facilitates the introduction of the thickened portion into the hole and can optionally be used for receiving displaced material.
If, according to another embodiment of the invention, the holes are provided with a cross-sectional increase in the insertion direction of the bristles behind the narrowest cross-section, then the space behind the narrowest cross-section can be used for the relaxation of the previously shaped thickened portion. This gives a reliable self-closure.
Another embodiment of the inventive process is characterized in that the edge of the holes is shaped onto the bristle carrier following the insertion of the bristles therein.
This in particular gives a surface free from holes or depressions, which provides an effective dirt protection. On smoothing the surface, a pressing action can also take place on any excessively forced back material of the thickened portions.
On the basis of the process defined hereinbefore, the problem of the invention can also be solved in that the thickened portions on the bristle ends and the wall of the holes of the bristle carrier comprising a thermoplastic is heated to a joining temperature between the transition temperature and the flow temperature for amorphous thermoplastics and to a joining temperature between the latter and the crystalline melting temperature in the case of partly crystalline thermoplastics or are kept at this joining temperature at which the thermoplastics are in the entropy elastic state and in this state the thickened portions are introduced into the holes and accompanied by the thermoforming of the wall of the hole, are bulged up outwards on the bottom of the hole.
Use is made of the same effects, in that following the bulging up of the thickened portion on the bottom of the hole, the hole wall is initially widened and then relaxed accompanied by the fixing of the deformed thickened portion.
In principle, any random geometry can be used for the bristle carrier. However, for the performance of the inventive process, a bristle carrier has proved advantageous, which is characterized in that it comprises a plate-like carrier part with sleeve-like projections projecting over the same on the bristle side or the back and that the holes on the bristle carrier receiving the bristles are located within the projections, at least over a larger part of the axial extension thereof.
In this embodiment, the wall of the hole can be more rapidly brought to the necessary joining temperature due to its smaller wall thicknesses than with a solid bristle carrier.